Thermal cracking method for producing olefines from hydrocarbons

ABSTRACT

In methods of manufacturing olefines by thermally cracking hydrocarbons, there is disclosed a thermal cracking method for producing olefines from hydrocarbons, characterized in that hydrocarbon in burnt with oxygen in the presence of steam to generate a high-temperature gas containing steam of 1500°-3000° C., methane and hydrogen are supplied into the high-temperature gas containing said steam, with the molar ratio of methane to hydrogen in said high temperature gas being 0.05 or more, then hydrocarbon to be cracked is supplied into said high-temperature gas containing said methane, hydrogen and steam, so that the hydrocarbon is subjected to thermal cracking by maintaining the partial pressure of hydrogen at more than at least 0.1 bar at the outlet of a reactor, under conditions of reaction temperature, 800°-1200° C. and residence time in the reactor 5-300 milli second, and then the reaction product is cooled in a rapid manner.

This is a continuation of application Ser. No. 581,256, filed Feb. 17,1984, now U.S. Pat. No. 4,527,002.

The present invention relates to a method of manufacturing olefines bythermal cracking of hydrocarbons, and more particularly, to a method ofmanufacturing olefines wherein hydrocarbon is burnt with oxygen in thepresence of steam to generate a high-temperature gas containing steam asa heat source for thermal cracking purposes, and methane and hydrogen inamounts required for reaction are supplied into said high-temperaturegas containing said steam so that the hydrocarbon can be thermallycracked in the presence of methane, hydrogen and steam.

It is already well known that the tubular-type thermal cracking method,the so-called steam cracking has heretofore been used as means forconverting light gaseous hydrocarbons such as ethane and propane andliquid hydrocarbons such as naphtha and kerosine to olefines.

In this kind of method, heat is supplied through a pipe wall fromoutside so that the speed of heat transmission and reaction temperaturesare limited and usually a reaction temperature less than 850° C. and aresidence time 0.1-0.5 second are adopted as reaction conditions.

However, under the restriction of such apparatus and reactionconditions, the materials for practical use are restrictively light oilsat best, but heavy oils such as residue oils cannot be subjected to theabovementioned conditions of restriction. The reason is that heavy oilsupon reaction at high temperature for a long time are sure to cause sidereactions of polymerization and condensation until coking occurs withthe result that the required gasification-rate of the heavy oil cannotbe attained.

As alternatives for such outer heating methods, there are severalmethods wherein a combustible gas of hydrogen or hydrocarbon is burntwith oxygen to produce a high-temperature gas, which may be utilized asa source for heating material hydrocarbon, thereby thermally crackingthe hydrocarbon.

The first method of the kind consists in burning methane and hydrogenobtained by cracking with oxygen to produce a high-temperature gas whichis mixed with steam to generate an atmosphere of high temperature,800°-1600° C. and material hydrocarbon is thermally cracked in theaforesaid atmosphere of high temperature under ordinary pressure for aperiod of residence time, 10-60 milli second, thereby manufacturing anolefine.

However, in such a method, the hydrocarbons for use as materials arelimited to light fraction less than light oil, and this method cannot beapplied to heavy oils because the rate of gasification is rather low anda coking condition occurs so enormously that no good result can beexpected of the method.

The second method consists in burning liquid hydrocarbon as fuel such ascrude oil to produce a high-temperature gas and thermally cracking amaterial hydrocarbon in said high-temperature gas under conditions ofpressure 5-70 bar, reaction temperature, 1315°-1375° C. and residencetime, 3-10 milli second. Yet, also in this method, inert gas such as CO₂and N₂ in film form is supplied into a reactor through a combustion zoneof said high-temperature gas whereby the occurrence of coking iscontrolled, thus enabling heavy oil such as residue oil to be thermallycracked.

In this method, it may be possible to prevent the adhesion of heavy oilto the wall of the reactor and to control coking due to thepolymerization of the decomposed gas at the wall surface, but it isimpossible to avoid coking, polymerization of decomposed gas and sootingtendency due to polymerization and condensation of the material oilitself, thus betraying the increase in yield of a useful gas. Moreover,in order to control coking, a pretty large amount of inert gas has to besupplied so that the charge on a refining and recovering system ofdecomposed gas may be increased and at the same time, thermal economy ofthe process becomes worsened as a drawback inherent in this method.

The third method consists in burning hydrogen in part to produce ahigh-temperature gas of hydrogen and manufacturing olefines from severalkinds of hydrocarbon including heavy oil in the above-mentionedatmosphere of hydrogen at reaction temperature, 800°-1800° C. underpressure of 7-70 bars, so that such reaction can be effected by rapidheating and thermal cracking can be attained in the above-mentionedatmosphere of hydrogen to eliminate coking whereby thermal cracking ofthe heavy oil material can be made possible.

However, still in this method, in the presence of a large amount ofhydrogen, the olefine of high value obtained by reaction arehydrogenated by said hydrogen, whereby it is converted to methane of lowvalue as a drawback in this method. Moreover, the formation of methaneby said hydrogenation is so exothermic that the temperature of reactionis elevated as the result of which such hydrogenation is accelerated andthe production of methane is further promoted with the tendency oframpant reaction. Thus it is causing a rapid decrease in ethylene and aincrease in methane production and making it difficult to maintain thepercentage of yield of olefine at a high level.

A more serious problem is that the production of methane is accompaniedby the consumption of valuable hydrogen until such valuable hydrogencauses ethylene to be wasted for manufacturing methane of low value, asa considerable loss in economy as another drawback inherent in thismethod.

In view of all the foregoing drawbacks, the inventors of the presentinvention have discovered that, firstly by arranging methane andhydrogen in coexistence, it is possible to prevent such a drawback asolefine becoming methane without losing the advantage of saidcoexistence of hydrogen and methane and to obtain a much higherpercentage of yield of ethylene than the conventional methods (JapanesePatent Application No. 038684/1982).

Namely, the inventors repeated strenuous researches for developing athermal cracking method of manufacturing olefines from varioushydrocarbons, which is capable of selectively attaining a highpercentage of the required olefines by preventing coking tendency in awide range from light hydrocarbons to heavy hydrocarbons. As the result,the inventors have discovered that hydrocarbon is burnt with oxygen inthe presence of steam to produce a high-temperature gas containingsteam, and material hydrocarbon is thermally cracked in the presence ofsaid high-temperature gas, methane and hydrogen, thereby obtaining ahigh percentage of yield of the required olefine not only from heavyhydrocarbons without fear of coking but also from light oils such asnaphtha, both in a high percentage of yield of olefine. Certainly basedon this discovery, the inventors have succeeded in achieving the presentinvention.

Namely, in methods of manufacturing olefines by thermally crackinghydrocarbons, the inventors propose a method of obtaining olefines fromhydrocarbons which comprises burning hydrocarbon with oxygen in thepresence of steam to produce a high-temperature gas containing steam ofhigh temperature, 1500°-3000° C., supplying methane and hydrogen withtheir mol ratio, more than 0.05 into the high-temperature gas containingsaid steam, than supplying material hydrocarbon to be cracked into saidhigh-temperature gas containing said methane, hydrogen and steam,thermally cracking said material hydrocarbon at temperature, 800°-1200°C. for a period of stay time, 5-300 millisecond by maintaining thepartial pressure of hydrogen at more than at least 0.1 bar at the outletof the reactor and then rapidly cooling the reaction product.

The thermal cracking method of the present invention will be explainedin detail hereinafter.

First of all, according to the present invention, the heat required forreaction can be supplied in the form of a high-temperature gas which isobtained by burning hydrocarbon with oxygen and since it is supplied byinternal heating, such high temperature can be easily obtained as isimpossible to obtain by external heating and in addition, a wastelessutilization of heat can be achieved. Although such internal heating byburning hydrocarbon has heretofore been proposed, gas-like hydrocarbonsand clean oil such as kerosine and light oil have been used in mostcases. Also, the procedure of using heavy oil has been proposed but uponcombustion of such material, there occurs the tendency of coking andsooting, thus requiring the circulation of a large amount of inert gassuch as CO₂ and N₂ already mentioned before.

According to the present invention, the combustion of fuel hydrocarbonis effected in the presence of a large amount of steam with a ratio 1-20(by weight) of steam to fuel hydrocarbon, including steam to be requiredat a later-stream reaction portion, whereby it is possible to controlcoking and sooting by dint of pacification of burning conditions andeffect of solid carbon reforming.

Besides, the amount of oxygen for supply may be more or less than thetheoretical equivalent concerned. However, particularly by supplying anamount less than the theoretical equivalent concerned for combustion, itis possible to prevent the consumption of hydrogen or losses ofeffective components due to the flow-out of unreacted oxygen to thelater-stream and to supplement hydrogen to be consumed during reaction.At the same time, CO may occur as a by-product but may be easilyconverted to hydrogen by shift reaction at the later stream and utilizedas a source of hydrogen.

Also, unlike CO₂, N₂ and other gases, the added steam can be easilycondensed for recovery in the step of separation and refining of crackedgas, thus providing the advantage of causing no increase in the chargeon the system of refining. In this case, according to the method of thepresent invention, such oxygen is used as could be obtained from air bydeep-freeze separation, diaphragm separation and absorbing separation.

With respect to the action of hydrogen, it has the following advantages.

Firstly, as compared with other materials, hydrogen has an extremelyhigh rate of heat transmission and consequently, it serves to heat evenheavy hydrocarbon in a rapid manner. Particularly in the case of heavyhydrocarbon for use as reaction material, hydrogen serves to control thereaction of polymerization and condensation as liquid-phase reaction byreducing the residence time in liquid phase condition so as to ensure ahigh percentage of gasification.

Secondly, it is possible to control the reaction of polymerization andcondensation in the aforesaid liquid phase by means of the activity ofthe hydrogenation and to supply hydrogen sufficiently as compared withthe content of carbon of heavy hydrocarbon, whereby the amount ofproduction of light gas is increased. Also against coke formation from agas phase, the amount of acetylene as a precursor of coke-formingreaction can be decreased for controlling purposes.

Thirdly, hydrogen has the effect of increasing the concentration ofradicals in the system of reaction, whereby high speed of cracking andgasification can be achieved. Indeed, these effects of hydrogen areespecially outstanding at high temperature under pressure conditions asproposed by the present invention.

However, it is true that hydrogen has a disadvantage which cannot beoverlooked. Namely, on account of presence of hydrogen and especiallyunder pressure, olefine may be wasted until a saturated product tends tooccur. This aspect amounts to nothing but a drawback inherent in thermalcracking methods in an atmosphere of hydrogen which has heretofore beenproposed. In other words, in an atmosphere of hydrogen alone, propyreneand ethylene arising from thermal cracking of material hydrocarbon arecaused to become hydrogenated in the following reactions (1)-(3).

    C.sub.3 H.sub.6 +H.sub.2 →C.sub.2 H.sub.4 +CH.sub.4 ( 1)

    C.sub.2 H.sub.4 +H.sub.2 →C.sub.2 H.sub.6           ( 2)

    C.sub.2 H.sub.6 +H.sub.2 →2CH.sub.4                 ( 3)

As the result, there occurs unavoidably the increase in methane andethane, and above all, the marked increase in methane. For one thing,all propyrene and ethylene may not disappear due to the fact that thespeed of reaction for their production is relatively rapid as comparedwith the above-mentioned reactions (1)-(3). So, even if quenching iscarried out to freeze these reactions, olefine may be lost due to thereactions (1)-(3) during such a short cooling time.

Another feature of the present invention is that not only hydrogen butalso methane is added to an atmosphere of reaction before itscommencement whereby it is rendered possible to control the hydrogenatedcondition of the reaction material even regarded as a drawback ofcoexistence of hydrogen without damaging the advantage thereof. Namely,upon adding an abundant amount of methane to the atmosphere of reaction,it follows that, simultaneously with the aforesaid reactions (1)-(3),there occur the following conversion reactions, (4)-(6) concurrentlywherein methane is converted to ethane and ethylene, thus preventing theconversion of the hydrocarbon material to methane by hydrogenation.

    2CH.sub.4 →C.sub.2 H.sub.6 +H.sub.2                 ( 4)

    C.sub.2 H.sub.6 →C.sub.2 H.sub.4 +H.sub.2           ( 5)

    C.sub.2 H.sub.4 +CH.sub.4 →C.sub.3 H.sub.8 →C.sub.3 H.sub.6 +H.sub.2                                                  ( 6)

In addition, by adjusting the reaction temperature, pressure and theratio of methane to hydrogen in the atmosphere of reaction, it ispossible to promote thermal cracking of methane until the added methanecan be converted to ethylene, ethane and acetylene of higher additivevalue.

For example, on the assumption that the reactions (4) and (5) forconversion of methane to ethylene are regarded as elementary steps ofreaction, the following reaction will take place. Namely, while a veryactive methyl radical (CH₃.) are produced from methane at hightemperature, this methyl radical changes into ethane by recombination,and moreover, there occurs a pull reaction of hydrogen or hydrogenradical (H.) until ethane is converted to ethylene either directly orthrough ethyl radical (C₂ H₅)

These reactions are formulated as follows. ##STR1##

This formation reaction of a methyl radical in the coexistence ofhydrogen and methane is shown as follows. ##STR2## Accordingly, in thepresence of a large amount of methane, the concentration of hydrogenradical will be decreased while the concentration of methyl radical isincreased. Namely, because methane becomes an absorbent for hydrogenradical, it can prevent the hydrogenation reaction of olefine byhydrogen radical, promoting the dehydrogenation reaction, and at thesame time, it show a function for converting methane to ethane andethylene by means of recombination of methyl radical having occurred ata time.

These effects of methane not only serve as a diluent but also methanecontributes very much to the increase in yield percentage of ethylene orothers from the viewpoint of reactive mechanism. Therefore, as comparedwith conventional cases where hydrogen is merely diluted with steam orinert gas, the function and effect of methane slow an enormousdifference and moreover, the abovementioned effect of methane can bedeveloped almost without reducing the advantage of hydrogen.

Again, according to the present invention, steam supplied to the burningportion can control coking also at the reaction portion byundermentioned water gas reaction, thus permitting the recovery ofvaluable hydrogen from heavy-material coking.

    C+H.sub.2 O→CO+ H.sub.2

or

    C+CO.sub.2 →2CO

CO can be converted to hydrogen by shift reaction) As a consequence, itbecomes possible to reduce the amount of hydrogen required for theatmosphere of reaction, so that the atmosphere of reaction turns outmild, and the hydrogenation of higher olefines such as propyrene andbutadiene can be controlled, which has heretofore been considereddifficult to achieve with methane alone, until the yield of propyreneand butadiene is increased and the consumption of hydrogen is reduced.

A preferred embodiment of the present invention will be explained indetail with reference to the accompanying diagrams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical view showing an example of preferredembodiment according to the present invention.

FIG. 2 is a graph showing the relationship between percentage of cokingyield and partial pressure of hydrogen.

FIG. 3 is a graph showing the relationship between yield (%) of C₂ -C₄olefine+ ethane and residence time (reaction time) on the basis of molratio of methane to hydrogen as parameter.

FIG. 4 is a graph showing the relationship between yield (%) of C₂ -C₄olefin+ ethane and pressure on the basis of mol ratio of methane tohydrogen as parameter.

FIG. 5 is a graph showing the relationship between percentage of productyield and temperature at the outlet of a reactor, and

FIG. 6 is a graph showing the relationship between yield (%) of ethyleneand yield (%) of C₃ +C₄ olefine and partial pressure of hydrogen.

Namely FIG. 1 shows an example of preferred embodiment with industrialapplication of the method of the present invention.

In the drawing, first of all, fuel hydrocarbon 1 is pressurized up tothe required extent by means of a pump 26 and the hydrocarbon thuspressurized is supplied to a combustion zone 2. To the combustion zone 2is supplied oxygen 4 of high concentration from oxygen-producing means 3and the fuel hydrocarbon 1 is burnt with said oxygen in the presence ofheated steam supplied from a line 5 to produce a high-temperature gasstream 6 of 1500°-3000° C.

In this case, there are some procedures for supplying steam alone or amixture of steam and oxygen or fuel or supplying it along the wall ofthe combustion zone 2 in order to protect the latter.

A high-temperature gas stream 6 issuing forth from the combustion zone 2is mixed with methane and hydrogen supplied from a line 29, theresulting mixture being permitted to enter a reaction zone 8. In thiscase, methane and hydrogen may be supplied individually or in mixedcondition. Also, these materials may be supplied almost to the sameposition as hydrocarbon material to be cracked or to the upstream of thereaction zone 8.

To the reaction zone 8 is supplied hydrocarbon material 7 by means of asupply pump 27, which has been pressurized up to the required extent.Then the hydrocarbon material 7 is brought into contact, and is mixed,with the high-temperature burning gas stream containing the aforesaidsteam, methane and hydrogen, and the resulting mixture is heatedrapidly. As the result, in a reaction zone 8 a thermal cracking reactionof the hydrocarbon material 7 take place under the action of theaforesaid hydrogen, methane and steam until a reaction fluid 9containing a large amount of olefine can be produced. This reactionfluid 9 is immediately conducted to rapid cooling means 10 and cooledrapidly therein. Said rapid cooling means 10 may be employed, forexample, in the form of direct rapid cooling means for spraying water oroil directly onto the reaction fluid, or an indirect rapid coolingheat-exchanger for heat exchange between two fluids inside and outsidethe pipe of said heat-exchanger, or a two-stage combination of theabove-mentioned two means.

Subsequently, the reaction fluid 11 after cooling is conducted to a gas-and liquid separator 12 so that it is separated into cracked gas andsteam 13 and cracked residual oil 14, said reside oil being utilized asa heat source for process steam.

On the other hand, cracked gas and steam 13 are cooled with water andafter condensing and separating the steam, it is conducted to separatingand refining means 15. In the separating and refining means, the crackedgas is separated into hydrogen and methane 16, olefines 17 such asethylene and propyrene, paraffins 18 such as ethane and propane and acomponent 19 heavier than C₄ component. Said separating and refiningmeans may be employed in the form of the usual deep-freeze separationmeans, absorbing means for separating methane and hydrogen fromcomponents more than C₂, or diaphragm separation means for concurrentuse. Paraffins 18 such as ethane and propane are conducted tosteam-cracking means 20 so that ethylene and propyrene can be recovered.

On the other hand, part of hydrogen and methane 16, if necessary, isseparated as methane product 21, after which the greater part thereof iselevated in pressure by means of a compressor 28 for hydrogen andmethane through a line 22 and may be recycled for purposes of reaction.Also, part thereof is permitted to enter a steam reformer and shiftconverter 24 through line 23, where methane is changed to hydrogen,which is then through a line 25 mixed with methane 22 as recycledhydrogen and the mixture is delivered to the reaction zone 8, so as tomake up for hydrogen which has been consumed during thermal crackingreaction.

Although omitted in FIG. 1, after cooling, the reaction fluid 11 andgases of H₂ S and CO₂ generated in the gas at the outlet of the reformermay be removed by gas refining means in the usual manner.

The hydrocarbons for use in the present invention may range from lightoils to heavy hydrocarbons such as various residual oils, shale oils,bitumen and liquefied coal oils and in some cases, slurry and solidhydrocarbons.

The characteristics of the present invention are that olefines can beeffectively manufactured from heavy oils by the combination of hydrogenand methane, while their treatment has heretofore been comparativelydifficult, that is to say, the present invention is entitled to becalled most effective in manufacturing olefines from heavy oils.

By the way, the fuel hydrocarbons for use in the present invention maybe anyone of the above-mentioned materials. Also, the hydrocarbons foruse as fuel and raw material to be cracked are not necessarily the same.Moreover, as fuel hydrocarbons, there is no restriction in selectinganyone from decomposed oils, undecomposed residual oils and off-gas froma separation and refining system.

As explained in detail hereinbefore, the thermal cracking method of thepresent invention has the following characteristics quite superior tothe conventional art.

Namely, hydrocarbon is burnt in the presence of steam under pressure toprovide heat required for reaction, and steam, hydrogen and methane arepurposely or selectively added to an atmosphere of reaction, whereby

(1) In the range of desired partial pressure of hydrogen (viz. partialpressure of methane) that a high percentage of yield of olefinecorresponding to hydrocarbon material to be cracked can be obtained, itis possible to control the harmful paraffin formation of olefine due tohydrogen with the use of methane. On the other hand, according to thepresent invention, it becomes possible to enhance the percentage ofyield of useful components such as olefine or the like from a wide scopeof hydrocarbon materials ranging from light hydrocarbons such as naphthato heavy hydrocarbons such as asphalt in a manner much better thanconventional methods. For example, in the case of use of asphalt as rawmaterial, the yield of olefine has heretofore been about 25%. Bycontrast, according to the present invention, it can be enhanced up toabout 45%. (2) In order to enhance the selectivity of percentage ofethylene yield (percentage of ethylene yield in percentage of olefineyield), partial pressure of hydrogen may be taken at a rather high levelwithin the aforesaid desired range of partial pressure of hydrogen. Atthe same time, in order to enhance the selectivity of percentage ofpropyrene and butadiene yield, partial pressure of hydrogen may be takenat a low level within the aforesaid range of partial pressure ofhydrogen so that the selectivity of products can be controlled. (3) Ascompared with light hydrocarbon materials, heavy hydrocarbon materialshave large content of polycyclic aromatic hydrocarbon which is difficultto crack, and therefore the required rate of gasification (ratio ofweight of decomposed hydrocarbon material to weight of hydrocarbonmaterial supplied to the reaction zone) is confirmed by maintainingpartial pressure of hydrogen at a relatively high level, after which theselectivity of products can be controlled by further adjusting partialpressure of hydrogen as described in the aforesaid Item (2).

(4) Since thermal cracking is carried out in the presence of steam,hydrogen and methane in combination, it is rendered possible to controlthe occurrence of coking more effectively than conventional methods. (5)Since it is possible to control paraffin formation from olefine due tohydrogenation by dint of methane in coexistence, the amount of producedolefine can be increased on one hand and the consumption of valuablehydrogen can be reduced in a proportional manner on the other hand. (6)As the exothermic effect along with the hydrogenation of olefine can beput under control, it is possible to obtain a distribution of yieldpercentage gradually variable against changes in reaction temperature,stay time and time of rapid cooling. This special effect is extremelyuseful in improving the operational and running properties of the plantconcerned. (7) In heavy hydrocarbons, the particulate property of oildrops in particular is important for enhancing the rate of gasificationand the percentage of yield of useful components as well as forcontrolling the tendency of coking. As means for achieving said purpose,it is necessary to increase the amount of high-temperature gas forhydrocarbon material, thereby increasing of the oil-drop shearing effectfor gas. Thereupon, unlike conventional methods, the present inventionemploys steam so that it can be recovered easily by cooling whereby theparticulate property of oil drops is promoted and its function isimproved without causing no charge on the system of refining.

(8) By burning fuel hydrocarbon in the concurrent presence of steam, itis possible to lower burning temperature and to control coking andsooting insomuch that heavy hydrocarbon like asphalt can be used asfuel.

(9) Since hydrocarbon is burnt with oxygen of high concentration, noinert gas is basically included so that there is little charge imposedon the system of separation and refining.

(10) By an operation of running under pressure, it is possible to reducepressure-elevation energy of cracked gas required for refining purposes.

In summary, according to the method of the present invention, it ispossible to attain a structure of the desired product at a high level ofyield from any hydrocarbon material without fear of coking.

Certain preferred embodiments will be further explained hereinbelow butthey are offered by way of mere illustration and not intended torestrict the present invention.

In these preferred embodiments, as the material and fuel for use, vacuumdistillation residual oil of Middle East (Specific gravity 1.02, S part43% and fluidized point, 40° C.) was used. First of all, steam waspreheated to 500° C. by a burner disposed above a reactor 2 and theaforesaid residue oil was burnt with oxygen of high concentration whichwas obtained by deep-freeze separation of air while blowing saidpreheated steam, to generate a high-temperature gas containing saidsteam. Subsequently, a gas of mixture of hydrogen and methane, afterpreheated at 400°-800° C., was blown onto the upper portion of thereactor 8 at downstream of the burner, so that it was mixed with saidhigh-temperature gas containing steam, and then a vacuume distillationresidue oil was sprayed into said high-temperature gas from a pluralityof asphalt burners disposed in the inner lateral walls of the reactorand after said residue oil was thermally cracked, the reaction productthus obtained was conducted to a cooler 10 provided in the lower part ofthe reactor 8 and water was blown directly to said reaction product, thereaction product thus treated being rapidly cooled and measured.

Also in this case, suitable range of residence time was obtained byreckoning the volume of the reaction vessel and reaction conditions.Likewise, naphtha (the range of boiling points 40°-180° C.) wasthermally cracked by the same method in the same apparatus and then thereaction product was measured. The ratio by weight of steam to fuelhydrocarbon was varied in each test, in order to ensure the requiredreaction conditions but the reaction was carried out within the range ofratio, approximately 0.5-30.

FIG. 2 is a graph showing the relationship between partial pressure ofhydrogen and percentage of yield of coking in the case of thermallycracking vacuume residual oil of Middle East and naphtha underconditions of temperature, 1000°-1020° C. at the outlet of the reactor,mol ratio 0.5 of CH₄ /H₂, total pressure, 30 bars and stay time, 20millisecond.

In the drawing, a indicates a curve showing the yield (%) of coke in thecase of thermally cracking vacuume residual oil of Middle East and bindicates a curve showing the yield (%) of coke in the case of thermallycracking naphtha. As is clear from FIG. 2, upon increase in partialpressure of hydrogen, the yield of coke is increased in an outstandingmanner and by maintaining partial pressure of hydrogen at a level morethan 1.5 bars in treating even heavy hydrocarbon such aspressure-reduced residue oil, the percentage of coking yield can becontrolled at an extremely low level.

Also in FIG. 2, an example of naphtha for use as light hydrocarbon isshown for purposes of comparison with heavy hydrocarbons. In this case,also by enhancing partial pressure of hydrogen, the formation of cokingcan be controlled and the effect of partial pressure of hydrogen can becalled more effective than for heavy hydrocarbons.

FIG. 3 is a graph showing the relationship between the yield (%) of C₂-C₄ olefine+ ethane yield based on the mol ratio of CH₄ /H₂ as aparameter and residence time, in the case of thermally cracking vacuumeresidue oil of Middle East as the reaction material under conditions ofpressure, 30 bars, temperature, 1000° C.-1030° C. at the outlet of thereaction vessel and total pressure, 30 bars. In this instance, the yieldof ethane in conformity to the yield of C₂ -C₄ olefine is evaluated forthe reason that the amount of the former is comparatively large and canbe easily converted to ethylene.

As is clear from FIG. 3, upon increase in the percentage of methane foraddition, the yield (%) of C₂ -C₄ olefine+ ethane is largely increasedand at the same time, changes in the percentage of yield for stay timebecome less and the distribution of yield-percentage is stabilized, allof which become well understandable. In this case, the higher the yield(%) of methane is, the larger is the proportion of C₃, C₄ components (C₃-C₄ olefine/C₂ -C₄ olefine+ ethane) present in the yield percentage ofC₂ -C₄ olefine plus ethane (ethane is 5-10%). Namely when the mol ratioCH₄ /H₂ is 1, the proportion of C₃, C₄ is 10-40% (the longer the staytime is, the smaller said proportion becomes.) Judging from theseresults, the addition of CH₄ leads to the obtaining of a higherpercentage of yield of olefine than in the case of no addition of CH₄(CH₄ /H₂ =0) shown for purposes of comparison and simultaneously,changes in the percentage of yield for residence time are remarkablyimproved. This effect of addition of CH₄ can be obtained also at the molratio 0.05 of CH₄ /H₂ but such effect is particularly outstanding atmore than 0.1. Moreover, it is well understood that residence time canbe selected in a manner of as wide as 5-300 millisecond.

FIG. 4 is a graph showing the relationship between pressure and theyield (%) of C₂ -C₄ olefine+ ethane in the case of thermally crackingvacuume residual oil of Middle East as the raw material under conditionsof temperature, 1000°-1020° C. at the outlet of the reaction vessel,residence time, 20 millisecond and the mol ratio CH₄ /H₂, 0 and 0.5. Asis clear from FIG. 4, while the influence of pressure on the yield (%)of C₂ -C₄ olefine ethane can be hardly perceived in the case of the molratio CH₄ /H₂, 0.5. But in the system of no addition of methane,according to increase pressure, methane is observed rapidly occurringupon cracking of C₂ -C₄ olefine and ethane by hydrogenation so that theyield of C₂ -C₄ olefine+ ethane is markedly lowered.

FIG. 5 is a graph showing the relationship between the temperature atthe outlet of the reaction vessel and the percentage of yield of C₂ -C₄olefine+ ethane in the case of thermally cracking vacuume residual oilof Middle East as the reaction material under conditions of totalpressure, 30 bars, residence time, 20 milli second and the mol ratio CH₄/H₂, 0 and 0.5. In the drawing, a indicates a curve showing therelationship between the yield (%) of C₂ -C₄ olefine+ ethane andtemperature at the outlet of the reactor under condition of the molratio CH₄ /H₂ 0 5, b indicates a curve showing the relationship betweenthe yield (%) of C₂ -C₄ olefine+ ethane in the case of mol ratio CH₄/H₂, 0, C is a curve showing the relationship between the yield (%) ofacetylene and the temperature at the outlet of the reaction vessel inthe case of mol ratio CH₄ /H₂, 0.5, and likewise, d is a curve showingthe relationship between the yield (%) of coking and the temperature atthe outlet of the reactor.

As is clear from the drawing, the yield (%) of C₂ -C₄ olefine+ ethane isas high as 40% in the case of mol ratio, 0.5 at 800°-1200° C. However,at less than 800° C., the speed of reaction is markedly lowered so thatthe yield (%) of C₂ -C₄ olefine+ ethane becomes largely lowered. On theother hand, at the side of high temperature, there occur a removal ofhydrogen from ethylene and the formation of acetylene by the cracking ofmethane, and such tendency becomes outstanding particularly at more than1200° C. so that the yield (%) of C₂ -C₄ olefin+ ethane is rapidlylowered. As a consequence, the amount of formation of coking isincreased which the polymerization and condensation of acetylene isconsidered to cause.

In the case of mol ratio CH₄ /H₂, 0, shown for purposes of comparison,namely in the case of no addition of methane, along with the rise oftemperature, there occurs progress of a rapid hydrogenation of C₂ -C₄olefine+ ethane until the percentage of yield of C₂ -C₄ olefine+ ethaneis largely lowered.

FIG. 6 is a graph showing the relationship between the percentage ofyield of ethylene or of C₃ +C₄ olefine and partial pressure of hydrogenat the outlet of the reactor in the case of thermally cracking vacuumeresidue oil of Middle East and naphtha under conditions of temperature,1000°-1020° C. at the outlet of the reaction vessel, total pressure, 10bars, residence time 15 milli second and mol ratio 0.5, CH₄ /H₂. In thedrawing, a indicates a curve showing the relationship between the yield(%) of ethylene and partial pressure of hydrogen in the case ofthermally cracking naphtha, b indicates a curve showing the relationshipbetween the yield (%) of C₃ +C₄ olefine and partial pressure of hydrogenin the case of thermally cracking naphtha, c indicates a curve showingthe relationship between the yield (%) of ethylene and partial pressureof hydrogen in the case of thermally cracking for vacuume residual oilof Middle East and d indicates a curve showing the relationship betweenthe yield (%) of C₃ +C₄ olefine and partial pressure of hydrogen in thecase of thermally cracking for vacuume residue oil of Middle East. As isclear from the drawing, in the case of thermal cracking of naphtha, thepercentage of yield of ethylene is increased along with the increase inpartial pressure of hydrogen but when partial pressure of hydrogen isfurther increased, propyrene and butadiene are decomposed to ethyleneand methane until the percentage of yield of C₃ +C₄ olefine isincreased. On the other hand, the percentage of yield of ethylene isfurther increased due to the contribution of thermal cracking of thecomponents of C₃ +C₄ olefine.

Also in the case of thermal cracking of vacuume residue oil of MiddleEast the influence of partial pressure of hydrogen is basically the sameas that in the case of thermal cracking of naphtha. Namely, along withthe increase in partial pressure of hydrogen, the percentage of yield ofolefine is increased on one hand and the percentage of yield of C₃ +C₄olefine is increased at the initial stage along with the increase inpartial pressure of hydrogen on the other hand, but when partialpressure of hydrogen is further increased, the percentage of yield of C₃+C₄ olefine is decreased due to its components being decomposed toethylene and methane.

Along with the increase in partial pressure of hydrogen, the trend ofchanges in the percentages of yield of ethylene and of C₃ +C₄ olefine isnoted to be the same in both cases of thermal cracking of naphtha andpressure-reduced residue oil of Middle East but the levels of partialpressure of hydrogen causing such changes in the percentage of yield aredifferent depending on the kind of hydrocarbon to be cracked. Namely inthe case of naphtha, more than 0.1 bar of partial pressure of hydrogenand in the case of pressure-reduced residue oil of Middle East, morethan 1.5 bars of partial pressure of hydrogen are preferable, in orderto obtain a high percentage of yield of olefine. Also by changingpartial pressure of hydrogen, it proves possible to control the rate ofthe yield (%) of ethylene or of C₃ +C₄ olefine present in the percentageof yield of the product.

Particularly when heavy hydrocarbon is used for cracking purposes andhydrogen present in hydrogen, methane and steam prior to being mixedwith the hydrocarbon material is less than 30 mol % (corresponding toless than about 3 bars of partial pressure of hydrogen) and also whenlight hydrocarbon is used for cracking purposes and the aforesaidhydrogen is less than 10 mol % (corresponding to less than about 0.8 barof partial pressure of hydrogen), a high percentage of yield ofpropyrene+ butadiene as C₃ +C₄ olefine proves obtainable.

Judging from the preferred embodiment, the scope capable of making thepresent invention effective can be defined as follows.

First of all, partial pressure of hydrogen is different depending on thekind of hydrocarbon to be cracked, namely, the heavier the hydrocarbonbecomes, the higher partial pressure of hydrogen is desirable. In otherwords, in the case of light hydrocarbon such as naphtha, more than 0.1bar of partial pressure of hydrogen is desirable and, in the case ofheavy hydrocarbon such as various kinds of residue oil, shale oil,bitumen, tar, liquefied coal oil, decomposed residual oil and petroleumcoke, more than 1.5 bars of partial pressure of hydrogen is desirable.

Next, concerning the percentage of methane for addition, in the case ofCH₄ /H₂ mol ratio less than 0.05, the effect of methane is slight andtherefore, more than 0.1 is preferable. On the other hand, the increasein CH₄ /H₂ mol ratio will require the increase in heat capacity forheating up to reaction temperature thus requiring the increase in energyunit. More than mol ratio 4 CH₄ /H₂ contributes quite little to theincrease in the percentage of yield of olefine and therefore,substantially less than mol ratio 4 is desirable. Likewise, in order toobtain a satisfactory percentage of yield of olefine, residence time forpurposes of reaction is 5-300 millisecond and preferably 10-100millisecond. At the same time, reaction temperatures 800°-1200° C. arealso desirable for obtaining a good percentage of yield.

What is claimed is:
 1. A thermal cracking method for producing olefinesfrom hydrocarbons which comprises burning hydrocarbon with oxygen in thepresence of steam to produce a high-temperature gas containing steam of1500°-3000° C., supplying methane and hydrogen into saidhigh-temperature gas containing steam, with the mol ratio of methane tohydrogen in said high temperature gas being more than 0.05, thensupplying hydrocarbon to be cracked into said high-temperature gascontaining said methane, hydrogen and steam, thermally cracking saidhydrocarbon under conditions of partial pressure of hydrogen of morethan at least 0.1 bar at the outlet of a reaction vessel at atemperature of 800°-1200° C. and at a residence time of 5-300 millisecond, and rapidly cooling the reaction product thus obtained.
 2. Athermal cracking method according to claim 1 wherein said steam of0.5-30 weight parts to one part of said fuel hydrocarbons is used.
 3. Athermal cracking method according to claim 1 wherein saidmethane/hydrogen mol ratio is from 0.05 to 4.0.
 4. A thermal crackingmethod according to claim 1 wherein said partial pressure of thehydrogen is at least 1.5 bar.
 5. A thermal cracking method according toclaim 1 wherein said residence time in the reactor is within the rangeof 10 to 100 milli-seconds.